1. Field of the Invention
The present invention concerns the field of manufacturing electronic components, especially integrated circuits, on substrates or wafers made of semiconductor materials, like silicon, and especially concerns methods and devices for identifying characters inscribed on a semiconductor wafer containing at least one orientation mark.
2. Description of the Art
The prior art teaches several methods and devices for identifying characters inscribed on semiconductor wafers. In particular, a process for identifying characters engraved on substrates in the form of a wafer is known. This process consists of using a manipulator arm to hold a wafer horizontally, the wafer being one of a plurality of wafers arranged to be aligned one behind the other in respective grooves of a vertically arranged carrier, and removing the wafer from the carrier oriented at an angle by means of an orientation mark on its perimeter in the form of a recess or flattened area so as to present a portion of the surface carrying characters to be identified in alignment with the optical axis of a camera and illumination system, especially the latter. This method has as a major drawback the need to remove the wafer from the support for the mere purpose of being identified, and also the drawback of requiring the presence of several elements above the wafer that are capable of contaminating it.
Another approach is known in which the angular orientation and identification of wafers arranged in the same carrier is possible using the same equipment, but here the camera and illumination systems are arranged above the wafers, causing significant air turbulence and thus increasing the risk of contamination of the wafers.
The present applicant has proposed methods and devices that permit elimination of these drawbacks by using a special geometry of illumination and observation of the characters on the semiconductor wafers being identified from below the wafers. Such methods and devices are described, in particular, in French Patent No. 2 711 824. However, these methods and devices, although fully satisfactory, require preliminary alignment of all the wafers in their support.
The prior art also teaches in published PCT Application No. WO 91/10968 methods and devices for identifying characters engraved on wafers using at least one mirror that reflects light rays used to identify the characters. In particular, this document describes an apparatus for identification of characters engraved on silicon wafers aligned in a carrier. In this apparatus, light radiation is reflected by one or two mirrors before illuminating the characters to be identified, and the image of the illuminated characters is then reflected by two mirrors before entering an observation camera. In order to illuminate the characters engraved on a wafer aligned in the carrier, the apparatus contains means to raise the wafer directly in front of the wafer being identified in order to permit placement of a mirror facing the characters. The mirrors are positioned at 45° relative to the plane defined by the wafer. This apparatus has the drawback of requiring manipulation of one wafer in order to permit identification of another wafer, which can cause a risk of contamination of the wafers. Another disadvantage is that identification time is slowed by these manipulations. Moreover, this apparatus uses several mirrors to observe the illuminated characters, which involves increased complexity of the mechanism and a higher selling cost of the apparatus. It can also cause identification errors of the wafers due to multiple reflections of the image on the mirrors before reaching the observation camera
The present applicant has proposed methods and devices for identifying the characters of semiconductor wafers that permits elimination of most of the aforementioned drawbacks and that advantageously uses a single reflection mirror. These methods and devices are described in French Patent No. 2 751 769. The methods and apparatuses described in this document permit effective identification of the characters inscribed on a semiconductor wafer without touching either the wafer to be identified or another wafer. This is owing to a specific geometry of illumination and observation, using a mirror to reflect incident light rays on the characters being identified, with the mirror being inserted between two successive wafers from below the wafers. Such methods and devices for identification also are entirely satisfactory. However, similarly to the methods and apparatuses described in the aforementioned French Patent No. 2 711 824, they require pre-alignment of the semiconductor wafers so that the characters being identified are first aligned.